1. Field of the Invention
The present invention relates to catalytic converters and methods for their use and preparation. More specifically, the present invention relates to improved catalytic converters which are useful in treating internal combustion engine exhaust.
2. Description of the Related Art
Catalytic converters are used in the exhaust system of automobiles to reduce the emission of pollutants such as hydrocarbons, carbon monoxide, nitrogen oxides and hydrogen sulfide. In a well tuned engine, incomplete combustion of gasoline tends to produce exhaust gas mixtures containing by volume 1 to 2% carbon monoxide, 500 to 1,000 parts per million (ppm) hydrocarbons, and up to a few thousand ppm nitrogen oxides. If allowed to pass untreated from a vehicle, hydrocarbons and nitrogen oxides mingle in the atmosphere, where photochemical reactions can convert them to smog. The other main component of exhaust pollution, carbon monoxide, is a deadly gas that can lead to asphyxiation or poisoning. Reduction in the emission of these compounds allows automobile operation with less strain on the environment and human health. Catalytic converters operate to reduce the emission of these compounds.
The most commonly used catalytic converter is a three-way converter that converts three harmful constituents of exhaust gas, namely carbon monoxide, nitrogen oxides, and hydrocarbons, to carbon dioxide, nitrogen gas, and water, respectively. The reaction is catalyzed by precious metal catalysts, which are supported on porous metal oxides.
The original metal oxide support was alumina (Al2O3). Recently, ceria (CeO2) also has also been used because it has an oxygen storage capability that makes oxygen available for the oxidation of hydrocarbons and carbon monoxide under certain oxygen limiting conditions of engine use. Ceria is often stabilized by mixing it with zirconia (ZrO2) to yield a homogenous mixed oxide.
Essentially, catalytic converters are composed of three partsxe2x80x94a ceramic or metallic substrate or monolith, a catalytic carrier or support, and a catalystxe2x80x94generally surrounded by a housing, such as a steel container, which holds the converter. Typically, the catalyst in a converter is a precious metal such as platinum, palladium, rhodium, or a combination of these metals. Generally, these metals are dispersed on washcoats, which are coatings that cover the substrate and increase the catalytic surface area and provide catalyst binding sites. The active substances in washcoats typically are inorganic materials, most commonly metal oxides.
It is well known that modern catalytic converters deteriorate with use. This deterioration is caused, in large part, by a reduction in area of the catalytic surface that gradually occurs with time under the extremely high temperatures of use. To compensate for this deterioration, catalytic converters are manufactured to have an excess of catalyst coated surface. This insures that as the catalytic converter ages and deteriorates, sufficient levels of catalyst will remain catalytically available to reduce engine emissions to acceptable levels. Due to the high cost of the precious metal catalysts, this strategy dramatically increases the cost of manufacture of catalytic converters.
Surface area stabilizers have been incorporated into catalytic converters during manufacture to increase stability. This strategy, however, does not significantly obviate surface area deterioration.
Thus, new catalytic converters with improved surface area stability are needed in order to control manufacturing costs and meet increasingly more rigorous emission standards. The present invention provides such catalytic converters. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides a catalytic converter for treating internal combustion engine exhaust comprising a substrate, fumed metal oxide aggregates adhered to the substrate, wherein no non-fumed metal oxide is present in the intra-aggregate voids, and at least one catalyst adhered to the fumed metal oxide aggregates. The present invention also provides a method of preparing such a catalytic converter, as well as a method of treating the exhaust of an internal combustion engine comprising contacting the exhaust of an internal combustion engine with a catalytic converter of the present invention.